Proton Therapy (PT) is a cancer treatment technology that uses high energy protons to penetrate a patient's body and deposit energy into treatment areas such as cancerous tumors. PT systems commonly implement a rotating gantry wheel that directs the proton beam to the patient from any angle between zero and 360 degrees. This allows the physician to design a treatment plan that attacks cancerous tumors from different angles and reduces radiation damage to critical organs and/or healthy tissue.
The charged protons may be generated in a particle accelerator, commonly referred to as a cyclotron and/or a synchrotron, and directed to the patient in the form of a beamline using a series of magnets that guide and shape the particle beamline such that the particles penetrate the patient's body at a selected location and are deposited at the site of the treatment volume. Particle therapy leverages the Bragg Peak property of charged particles such that the majority of the energy is deposited within the last few millimeters of travel along the beamline—at a point commonly referred to as the isocenter, as opposed to conventional, intensity modulated radiation therapy (i.e., photons) in which the majority of energy is deposited in the first few millimeters of travel, thereby undesirably damaging healthy tissue.
Particle therapy treatment facilities typically consist of a single cyclotron and a plurality of treatment rooms. Thus, the single cyclotron is often adapted to generate a particle beamline that is then selectively directed to one of the various treatment rooms. A particle therapy treatment may include the selection of a desired energy level for the beamline, such that the energy of the particles is deposited substantially at the desired location (i.e., the treatment volume) inside the patient's body. Therefore, the energy level selection is directly related to the position and shape of the treatment volume within the patient's body. Frequently, the cyclotron will generate a standard high-energy beamline, which may then be selectively modified as desired for the particular treatment protocol.
The beamline may be directed immediately to the patient without the need for any redirection. However, a more common approach is to redirect the beamline using a series of cooperating bending magnets which route the beamline to a proton nozzle mounted on a gantry. FIG. 1 illustrates an example embodiment prior art particle therapy gantry designed to receive and redirect a particle beamline to a patient. As illustrated, the particle therapy gantry 21 includes at least three bending magnets 11A-C to redirect the particle beamline 15 to the gantry's treatment nozzle 13, and eventually the patient 9 positioned on a treatment bed 17. This allows the beamline 15 to be selectively directed to the patient 9 from any angle and permits a physician to design a treatment plan that minimizes undesirable effects on healthy tissue. Stated differently, gantries are frequently adapted to rotate about a patient, and redirect the beamline to be perpendicular to the gantry's axis of rotation 19, illustrated by the directional arrow 19′ in FIG. 1. Thus, the treatment nozzle 13 and beamline 15 may be rotated about the patient 9 such that the beamline 15 is able to penetrate the patient's body at a plurality of locations and encounter the treatment volume from multiple directions. This minimizes adverse effects on healthy tissue and increases the efficacy of the treatment.
Thus, in comparison to standard x-ray therapy, proton therapy is capable of significantly improving dose localization by increasing the dose delivered to the target volume, while minimizing the dose delivered to the surrounding tissue. These improvements are based on the finite penetration range of therapeutic proton beam in the target material. Furthermore, energy deposition to the target material increases as the proton beam slows down and reaches maximum energy near the end of the penetration range. The penetration depth and the location of the energy deposition peak (the Bragg peak) are defined by the proton beam energy. Therefore, a proton beam of a given energy delivers a therapeutic dose of energy at a specific treatment depth. In order to deliver this therapeutic dose to a target with a given extent in depth, proton beams with several different energies can be used.
To provide these several different energies, older proton therapy systems typically changed the proton beam energy in the treatment nozzle by inserting plates of material that attenuate the proton beam energy by the specified amount. More advanced conventional systems change the beam energy further upstream near the proton accelerator itself. Such conventional systems typically require a change of the beam transport line to match each subsequent proton beam energy. Thus, a proton therapy system in which proton beam energies can be changed without such structural changes and adjustments as described above would be desirable.
Another of the challenges facing PT systems is to maintain proper alignment between the proton delivery nozzle and the isocenter of the rotating gantry system when the gantry is rotated to different treatment angles. For example, it is desirable to maintain accuracy of the proton beam to the gantry center when the gantry apparatus is rotated to different positions in order to accurately focus the proton beam to a targeted area of interest. Due to inherent fabrication tolerances and the extreme size and weight of the gantry apparatus and its various components, the structure can deflect when rotated at different angles, allowing the system's center to drift above the target accuracy.
It is known to move the patient bed to compensate for subtle drifts in the system at different angles of rotation. However, moving the patient to compensate for beam misalignment can become quite time consuming and complicated, especially if the treatment plan requires more than one application angle for each patient. Therefore, it is desirable to provide a proton therapy system having a smaller and lighter gantry wheel to avoid some of the structure based deflection.